Air Quality

Increasing Risks from Air Pollution

More than 100 million people in the United States live in communities where air pollution exceeds health-based air quality standards. Unless counteracting efforts to improve air quality are implemented, climate change will worsen existing air pollution levels. This worsened air pollution would increase the incidence of adverse respiratory and cardiovascular health effects, including premature death. Increased air pollution would also have other environmental consequences, including reduced visibility and damage to agricultural crops and forests.

Unless offset by additional reductions of ozone precursor emissions, there is high confidence that climate change will increase ozone levels over most of the United States, particularly over already polluted areas,3,86 thereby worsening the detrimental health and environmental effects due to ozone. Although competing meteorological effects determine local ozone levels, temperature is often the largest single driver.87 The climate penalty35,36 results from changes in local weather conditions, including temperature and atmospheric circulation patterns,4,88 as well as changes in ozone precursor emissions that are influenced by meteorology.75,76,77 Climate change has already had an influence on ozone concentrations over the United States, offsetting some of the expected ozone benefit from reduced precursor emissions.89,90 Assessments of climate change impacts on ozone trends are complicated by year-to-year changes in weather conditions91 and require multiple years of model information to estimate the potential range of effects.92 Besides being affected by climate change, future ozone levels in the United States will also be affected greatly by domestic emissions of ozone precursors as well as by international emissions of ozone precursors and global methane levels. Studies suggest that climate change will decrease the sensitivity of regional ozone air quality to intercontinental sources.93

PM2.5 accounts for most of the health impacts due to air pollution in the United States,94 and small changes in average concentrations have large implications for public health. Without consideration of climate effects, concentrations of PM2.5 in the United States are projected to decline through 2040 due to ongoing emissions control efforts.34 PM2.5 is highly sensitive to weather conditions, including temperature, humidity, wind speed, and rainfall. The effects of climate change on the timing, intensity, duration, and frequency of rainfall are highly uncertain, influencing both the removal of PM2.5 from air and the incidence of wildfires and their associated emissions. Accordingly, the net impact of climate-driven weather changes on PM2.5 concentrations is less certain than for ozone.3,4,43,70 However, some studies have indicated that even without considering increased wildfire frequency, climate change will cause a small but important increase in PM2.5 over North America.95,96 The impact of climate change on the PM2.5 contribution from intercontinental sources, which depends strongly on projected changes in precipitation, remains highly uncertain.24

Figure 13.2: Projected Changes in Summer Season Ozone

Four maps of the contiguous United States. The top two maps display projected changes in average daily summertime ozone concentration levels (in parts per billion) for the year 2050 under a lower scenario (RCP4.5) and a higher scenario (RCP8.5). The bottom two maps display this projected change at 2090 for the lower and higher scenarios. Under the lower scenario for 2050, ozone concentration is projected to rise across much of the Nation, including the Southwest, Northern Great Plains, Midwest, and Northeast. Changes are most pronounced in the Northern Great Plains and the Midwest, and also along the Great Lakes coastline where concentrations could rise by 4 to 5 parts per billion. Ozone concentration is projected to decrease slightly in the Pacific Northwest, across the southernmost United States, and in the state of Maine, with the most pronounced decrease (2 parts per billion) projected for South Texas and South Florida. These changes are amplified under the higher scenario (RCP8.5) and also for 2090 as compared to 2050.

Figure 13.2: The maps show projected changes in summer averages of the maximum daily 8-hour ozone concentration (as compared to the 1995–2005 average). Summertime ozone is projected to change non-uniformly across the United States based on multiyear simulations from the Community Multiscale Air Quality (CMAQ) modeling system. Those changes are amplified under the higher scenario (RCP8.5) compared with the lower scenario (RCP4.5), as well as at 2090 compared with 2050. Data are not available for Alaska, Hawai‘i, U.S.-Affiliated Pacific Islands, and the U.S. Caribbean. Source: adapted from EPA 2017.1

The health impacts of climate-induced changes in air quality may be reduced by various adaptation measures. For example, as local authorities issue air quality alerts, people may reduce their exposure to air pollution by postponing outdoor activities and staying indoors (for further information on the role of adaptation in reducing climate-related health risks, (see Ch. 14: Human Health, KM 3).

The magnitude of the climate penalty over the United States could be reduced by mitigating climate change.1,90,97 For example, Figure 13.2 shows results from one study1 projecting the change in summertime ozone resulting from two different future scenarios (RCP8.5 and RCP4.5) see the Scenario Products section of App. 3 for additional information about these scenarios) at 2050 and 2090, with human emissions of ozone precursors held constant. Due to climate change, ozone is projected to increase over a broad portion of the United States. Mitigating climate change globally (for instance, following RCP4.5 rather than RCP8.5) would reduce the impact on ozone, resulting in fewer adverse health effects, including 500 fewer premature deaths per year due to ozone in 2090.1